A wide variety of elongated medical instruments that are adapted to be permanently or temporarily implanted in the mammalian body, usually the body of a human patient, or used to access a site in the body to facilitate introduction of a further medical device. Such elongated medical instruments have an instrument body extending between instrument body proximal and distal ends, and a distal segment of the instrument body is advanced to a remote site in the body by manipulation of a proximal segment of the instrument body or a handle or stylet or the like extending from the instrument body proximal end located outside the body.
Elongated medical instruments include implantable medical electrical leads, catheters, sheaths, endoscopes, guidewires, and the like. In the case of a medical electrical lead, the lead body proximal end is coupled to an implantable pulse generator (IPG) of an implantable cardioverter/defibrillator (ICD) or pacemaker or to a monitor that is then implanted subcutaneously or to an external medical device located outside the body and electrical signals are conducted to or from the remote site in the body through one or more lead conductors. Catheters typically extend through the patient's skin and are coupled with external diagnostic or therapeutic equipment or are used to introduce other elongated medical instruments or fluids or cells or proteins or the like, or to withdraw fluids or measure pressure, or the like, through a catheter lumen open at the accessed remote site. Certain catheters, e.g., electrophysiology ablation and mapping catheters, also deliver electrical energy or conduct electrical signals of the body. Other catheters include pulmonary artery catheters, central venous catheters, diagnostic coronary catheters, intra-aortic balloon pump catheters, balloon tipped (PTCA)/angioplasty catheters, and stent delivery catheters. The terms “catheter” and “lead” are often interchanged in these and other contexts. Guidewires are small diameter wires that are directed through tortuous pathways to provide for advancement of medical leads or catheters over-the-wire. Certain guidewires are also designed to function as a micro-catheter for infusion of fluids through a guidewire lumen. Other guidewires include insulated electrical conductors connected at the guidewire proximal end with an external medical device to deliver electrical energy for tissue stimulation or to conduct electrical signals of the body to the external medical device. Hence, in the following discussion, the terms electrical medical leads, catheters and guidewires comprise and can be used interchangeably with elongated medical instruments.
In many cases, the introduction of such elongated medical instruments to a remote site in the body is effected through a needle (Seldinger Technique) or skin incision accessing a blood vessel, whereby the instrument body is advanced through a vascular pathway until the distal segment or the instrument body distal end are located at the remote site. Such advancement is often through a tortuous pathway having twists and turns requiring the capability to impart a curve or deflect the instrument body distal end to facilitate advancement. Therefore, the introduction of such elongated medical instruments through vascular pathways or other tortuous pathways in the body is facilitated by a wide variety of techniques and mechanisms that have been developed to impart curves in the distal segment of the instrument body or to deflect the instrument body distal end.
One typical approach facilitating catheter introduction involves one or multiple needles, guide wires, dilators and hemostatic sheaths. The needle gains access to the vasculature (vein or artery), and a guidewire is inserted through the needle for pathway retention. The needle is removed. A dilator is inserted into a hemostatic sheath, which can then be advanced over the retaining guidewire and into the pathway. The guidewire and dilator are removed. The sheath can be advanced closer to target locations. A guidewire or guide catheter can be inserted into the hemostatic sheath for access retention or further steering functionality. In this case, a catheter lumen terminates in a distal lumen end opening allowing the catheter to be inserted over the proximal end of the guidewire outside the body. Another approach employed with closed end catheters and medical electrical leads involves preforming a curve into the distal end segment of a stiffening stylet and inserting the stylet into a catheter or lead lumen to curve or deflect the corresponding distal segment of the catheter or lead. The distal segments of certain stylets and guidewires can alternatively be deflected within the lumen by operating a shaping mechanism of the types described below at the proximal end of the stylet or guidewire outside the body.
One or more pull wire or push-pull wire mechanisms are commonly employed to provide controllable deflection of the distal end segments of catheters and guidewires. Typically, the catheter, or guidewire is formed having a generally straight outer sheath and a pull or push or push-pull wire extending through a lumen of the outer sheath to an attachment point at the sheath distal end. The wire is pushed or pulled on at its proximal end typically through a handle that is permanently or removably attached to the catheter or guidewire proximal end. The proximal retraction or distal advancement of the pull or push wire, respectively, causes a distal segment of the outer sheath controlled to bend or deflect. Various mechanisms are employed to control the direction of deflection of each segment.
In another approach, one or more piezoelectric member or shape memory material member, particularly a shape memory alloy (SMA) member, that bends from a first shape or direction to a second shape or direction upon application of electrical energy to the member is incorporated into a deflectable segment of a catheter, medical electrical lead or guidewire. The characteristics of SMA materials and particular examples of SMA members incorporated into catheter or guidewire bodies are described in the literature and in a number of patents including U.S. Pat. No. 3,890,977 to Wilson, U.S. Pat. No. 4,799,474 to Ueda, U.S. Pat. Nos. 4,918,919 and 5,055,101 to McCoy, U.S. Pat. No. 6,306,141 to Jervis, and U.S. Pat. Nos. 6,133,547 and 6,278,084 to Maynard. SMA members are also referred to as actuators or elements that are activated in certain of the above-referenced patents.
As described in the above-referenced '547 patent, SMA material undergoes a micro-structural transformation from a martensitic phase at a low temperature to an austenitic phase at a transition temperature. When in the martensitic or low temperature phase, SMA exhibits low stiffness (low elastic modulus) and may be readily deformed up to 8% total strain in any direction without adversely affecting its memory properties.
Upon being heated to its activation temperature, the SMA becomes two to three times stiffer as it approaches its austenitic state. The higher elastic modulus, at the higher temperature, is the result of the SMA reorganizing itself on the atomic level to a body-centered cubic (BCC) crystal structure to return to a previously imprinted or “memorized” shape, if unrestricted. Useful motions and forces may be extracted from a SMA element as it attempts to move to its previously memorized shape. If permitted to cool below the transformation temperature, the BCC crystal structure goes through a diffusionless shear transformation to a highly twinned martensite crystal structure. A shape may be “trained” into a SMA by heating it well beyond its activation temperature to its annealing temperature for a period of time. In one example for a TiNi (sometimes referred to as Nitinol) SMA system, the annealing program consists of geometrically constraining the specimen, and heating it to approximately 510° C. for fifteen minutes.
The point at which a SMA becomes activated is an intrinsic property of the material and is dependent on stochiometric composition. A change in alloy ratios of 1% produces a 200° C. shift in transition temperature, for a typical SMA such as TiNi (49:51). Binary TiNi SMAs can have a large range of transition temperatures. For Nitinol, atomic composition can be adjusted for a phase transition as high as 100° C. and as low as −20° C. or more. Sub-zero transition materials exhibit superelastic behavior. That is, they can reversibly endure very large strains at room temperature. In the medical community, superelastic formulations of Nitinol are commonly employed in “steerable” guidewires or catheters.
In contrast to the passive characteristics of a superelastic SMA, a SMA actuator that must perform work on its environment requires a SMA capable of producing useful forces and motions for a given input of thermal energy. Because most thermal devices must expel their waste heat to the ambient environment, which in most cases is near room or body temperature, higher transition point SMAs are most commonly used as active actuator elements. During phase changes, a SMA will exhibit a maximum recoverable strain of up to 8%. In general, electrical energy is applied to a SMA member or to a resistive heating element adjacent to the SMA member to cause the SMA member to heat up and bend in the second direction. The bend in the second direction is maintained until the energy is removed and the SMA member cools, whereupon it bends back toward the first direction. Such SMA members are also referred to as actuators or elements that are “activated” when heated by applied energy in certain of the above-referenced patents.
The SMA element is typically formed of an elongated strip of SMA element disposed to extend lengthwise along the catheter segment as disclosed in the above-referenced '977 patent. The catheter segment is bent to the second direction upon application of electrical current directly to the elongated SMA member to heat it. SMA materials do not fully return to the first shape or direction after cooling, and other mechanisms (externally applied forces) are required to restore the first shape or direction. The restoration can be accomplished actively or passively through further components incorporated into the segment of the catheter. For example, a return spring can be incorporated into the segment that is just strong enough to fully deflect the SMA member in the first direction in its (cool) martensitic state. The SMA member exerts enough force when activated to overcome the passive return spring force and bend or deflect the segment of the catheter in the second direction. The passive return spring force bends or deflects the segment back in the first direction when the SMA member is de-activated. Thus, the forces work in opposition to one another in the plane of the induced bend.
In an active or antagonistic configuration as disclosed in certain embodiments of the above-referenced '101 and '474 patents, first and second elongated SMA elements or members are captured by a common catheter segment that is to be deflected. The first and second elongated SMA members are captured by the catheter sidewall such that they extend in parallel with one another and the catheter axis and are displaced 180° apart around the sidewall, that is, diametrically across the catheter diameter. When the first SMA member is heated to its activation temperature, it provides sufficient force to deflect itself, the second SMA member, and the catheter sidewall segment in the first direction. When the second SMA member is heated to its activation temperature, it provides sufficient force to deflect itself, the first SMA member, and the catheter sidewall segment in the second direction. Again, the forces are diametrically opposed or antagonistic to one another such that the activated SMA member must apply force to bend the inactivated (martensitic or soft) SMA member from its proximal end or root disposed diametrically across the diameter of the catheter shaft as well as the catheter sidewall itself.
In the above-referenced '474 and '919 patents, the electrical current is applied directly through conductors attached at opposite electrical ends of the SMA member. The SMA member is formed as either an elongated bar of SMA material, so that the opposite electrical connection ends are located at different distances along the length of the catheter shaft, or a U-shaped split bar of SMA material, so that the opposite electrical connection ends are located at a common point along the length of the catheter shaft. As noted in the '919 patent, the attachment of electrical conductors to the opposite ends of the SMA member contaminates the SMA alloy and negatively affects performance, and steps are disclosed for isolating the contamination areas. In either case, the applied electrical current directly heats the SMA member that responds by bending along its length in the non-contaminated area by an amount that depends on composition, dimensions, and heat treatment of the SMA member.
In the '084 patent, various embodiments of a SMA actuator are disclosed comprising a SMA member that is electrically insulated from a plurality of resistive heating elements deposited or formed overlying and in close proximity to the SMA member. The SMA member is preferably formed as a two-dimensional strip or sheet of SMA material that can bend in one or two dimensions. Directing electrical current to selected resistive heating elements overlying the discrete portions effects the bending of discrete portions of the SMA member. A flexible layer of insulation is deposited over the SMA strip or sheet, and the resistive heating elements and conductive traces from bond pads to the resistive heating elements are formed on the electrically insulating layer. In this way, direct electrical connection with and application of current to the SMA material is avoided, and discrete portions of the SMA material can be activated. Moreover, micro-machining and integrated circuit fabrication techniques can be employed to form SMA actuators in a wide array of configurations.
In one disclosed configuration, a tubular SMA actuator is formed as described above and mounted along the shaft of a catheter or a guidewire or the like. Longitudinally extending, parallel slits are formed in the tubular SMA actuator, thereby providing a plurality of longitudinally extending “finger-like segments” extending from a common cylindrical band encircling the catheter shaft. Each finger-like segment comprises at least one or a plurality of resistive heating elements and conductive traces deposited as described above. Thus, each finger-like segment comprises discrete portions of SMA material that are selectively bendable, whereby the entire finger-like segment is bendable to a selective degree depending upon selective current flow through the discrete resistive heating elements. This system could work in theory, but practically would be difficult to reduce to practice.
U.S. Pat. No. 6,072,154 (FIGS. 24A-24C) depicts incorporation of the tubular SMA actuator into a catheter shaft and selectively activating the finger-like segment along one side as described above to enable selective bending of the catheter shaft as the finger-like segment bends outward and pulls the inactivated finger-like segments along in the direction of the bend. Presumably, the catheter shaft could be bent in any direction away from the shaft axis through selective activation of any of the finger-like segments arrayed through 360° about the axis. But, the activation energy required to actuate any one or more finger-like segment and pull along the remaining inactivated finger-like segments disposed around the catheter shaft would appear to be high. The resistance of multiple inactive finger-like segments would make it difficult to bend the catheter shaft by activating any of the finger-like segments.
While the mechanisms disclosed in the above cited prior art patents may at least to some degree be workable, there is still a need for a SMA deflection mechanism that is simple, inexpensive to manufacture, does not excessively increase the elongated medical instrument body diameter, and reduces the bending forces and stress/strain that are necessary to deflect the distal segment of the catheter body while enhancing the control of the deflection and the imparted curvature of a catheter body distal segment.